Various methods are known for locally depositing a metal film on a transparent substrate, these methods having been used to repair clear defects in photomasks which are used in the manufacture of semiconductor devices. One known method includes the step of depositing a photoresist over the entire surface of the photomask containing the defect. The defective area is then exposed to ultraviolet radiation, after which the photoresist is developed to dissolve the material where it has been exposed to the radiation. The substrate is then placed in a vacuum chamber in which chromium is evaporated to deposit chromium on the surface of the substrate. The photoresist is removed leaving a chromium deposit on the area exposed to the ultraviolet radiation. This method for repairing a photomask is not only time-consuming and of limited resolution, but further results in chromium deposits in undesired areas thus creating additional defects in the photomask which must be removed.
One known method for depositing a metal film on a substrate as shown in the U.S. Pat. No. 4,340,617 employs an ultraviolet laser which is focused onto a substrate positioned in a gas cell into which metal bearing gases are introduced. The gas absorbs a portion of the laser energy for effecting photodecomposition of the gas near the surface of the substrate to deposit metal material on the substrate. This method is undesirable because ultraviolet laser sources are typically very large in size, bulky and have been found to be unstable when operating continuously for a given period of time. The optics used with such lasers are also very difficult to adjust resulting in problems with focusing and resolution. Further, photodeposition rates are typically low.
Another known method for depositing a metal film on a substrate is a laser-induced thermal deposition process. One such process employs a CO.sub.2 laser, generating infrared radiation, to deposit a metal film on a quartz substrate. This method has been found undesirable, however, because the resolution is limited essentially to the wavelength of the radiation; namely, 10 .mu.m and the complications involving far infrared optics. Another known thermal deposition process employs a visible laser to deposit a metal film on an opaque substrate. Problems have arisen with this process, however, when the visible laser is employed to deposit a metal film on a transparent substrate. It has been found that it takes a very long time to heat a clean transparent substrate to the point at which the metal bearing molecules decompose on the hot substrate surface and stick thereto to form a metal film on the substrate surface. Typically, the visible laser heats the transparent substrate a little, causing the molecules to decompose, however, the metal film does not immediately stick to the substrate but evaporates. After heating the substrate a sufficiently long period of time, on the order of 2-3 seconds, a metal deposit will form on the substrate surface, the deposit being much larger than the area of the substrate to which the visible laser is incident. There is very little control over the size of the metal deposit produced by this method so that the deposits are not reproducible. Further, very small deposits on the order of a micron cannot be made with this method.